Placeholders should popup a new modal/form to insert them

[GaurangTandon]

Attempting to insert placeholders in content editable nodes causes more problems than imaginable.

1. Not all CE nodes support having spans with custom classes and therefore will remove those classes, ex: Slack, so we won't detect anything.
2. Being able to support custom form like behavior is impossible through such approach.

Therefore, I suggest opening a popup. The popup may not be a new window, but rather simply a modal dialog similar to that for blocking site.